fix close button in system menu for AboutDialog on Windows


If SetEscapeId receives wx.ID_CLOSE as parameter, then on Windows with
wxPython 2.8 system close button in title bar stops working.
If wx.ID_CANCEL or wx.ID_OK is used in SetEscapeId call, then close
button works correctly.

/**
 * No platform specific code for "Generic" target
 **/